1. Field of the Invention
The present invention relates to an optical disk recording/reproducing apparatus for recording and reproducing information on and from an optical disk, and more particularly to a servo system for controlling the position of an objective lens in an optical pickup.
2. Description of the Related Art
Conventionally, an optical disk recording/reproducing apparatus has an objective lens for converging a laser to a track of an optical disk, and also has an optical pickup for optically performing recording/reproducing of information on an optical disk, a tracking servo system for tracking-controlling the objective lens according to a tracking error signal generated from an RF signal received from this optical pickup, and a focusing servo system for performing a focusing control operation on the objective lens according to a focusing error signal generated from an RF signal received from the optical pickup.
The tracking servo system has a tracking error detection circuit for detecting a tracking error signal generated from an RF signal received from the optical pickup, a tracking servo filter for appropriately adjusting the frequency characteristic of the detected tracking error signal, a tracking servo amplification circuit for amplifying the tracking error signal received from this tracking servo filter, and a focusing servo drive circuit for outputting a drive signal that is used for driving a tracking servo actuator for performing a tracking position control operation on the objective lens according to a signal outputted from this tracking servo amplification circuit.
The focusing servo system has a focusing error detection circuit for detecting a focusing error signal generated from an RF signal received from an optical pickup, a focusing servo filter for appropriately adjusting the frequency characteristic of the detected focusing error signal, a focusing servo amplification circuit for amplifying the focusing error signal received from this focusing servo filter, and a focusing servo drive circuit for outputting a drive signal that is used for driving a focusing servo actuator for performing a focusing position control operation on the objective lens according to a signal received from this focusing servo amplification circuit.
Meanwhile, in a case where a laser beam converged by the objective lens of the optical pickup shifts from a central position of a track of an optical disk, that is, in a case where the lens shift of the objective lens is performed, a tracking servo is applied by performing feedback control, under which a tracking servo actuator is controlled to thereby correct the lens shift of the objective lens and return the objective lens to an original position (that is, a position at which the optical pickup can irradiate laser beams from the objective lens to the center of a track), in the tracking servo system.
However, in the case where the lens shift of the objective lens occurs, the sensitivity of the tracking servo actuator is reduced according to the shift amount of the objective lens from the lens central position a, as shown in, for example, FIG. 2. This results in occurrences of problems that the controlling force of the tracking servo is lowered and that stable tracking servo cannot be applied.
Further, in a case where the laser beam converged by the objective lens of the optical pickup shifts from a focal position on a track of the optical disk, that is, in a case where the lens shift of the objective lens is performed, a focusing servo is applied by performing feed back control, under which a focusing servo actuator is controlled to thereby correct the lens shift of the objective lens and return the objective lens to an original position (that is, a focal position, at which the laser beams are converged, on the track), in the focusing servo system.
However, in the case where the lens shift of the objective lens occurs, the sensitivity of the focusing servo actuator is reduced according to the shift amount of the objective lens from the focal position c as shown in, for example, FIG. 4. This results in occurrences of problems that the controlling force of the focusing servo is lowered and that stable focusing servo cannot be applied.
Incidentally, although the prior art described in JP-A-5-73946 corrects the lens shift by adjusting only one of a tracking adjusting portion, which includes a relay lens driving coil for tracking-controlling, and a drive portion including a driving coil of a linear motor serving as a movable portion driving means thereby to set a boundary point and a gain. However, this prior art takes no measures against the reduction in the controlling force of the tracking servo in a state in which the lens shift is performed. Thus, this prior art cannot solve the aforementioned problems.
Although the prior art described in JP-A-11-25474 performs traverse control so as to reduce a lens shift amount, this prior art takes no measures against the reduction in the controlling force of the tracking servo in a sate in which the lens shift is performed. Thus, this prior art cannot solve the aforementioned problems.